Jonathan is 14.11 years and in Grade 8. His understanding of basic concepts mystifies me. He is left handed and if he adds 135, 6 and 72 he gets 1455. i got him to count by 2s and he got to 24 correctly; then he  jumped to 28,32,35, 44. Counting by 3s was worse. Ask him how many 2s in5 and his answer is one. What can I do to help him to understand basic concepts- especially the four operations? He is not always this way. Sometimes he gets all of his work correct or at least a B. On other days, he may be D or F and need extra help.Any suggestions?